It is not easy to measure the flow of electricity accurately over a wide dynamic range while dissipating very little waste heat.
Whenever electrical energy is used, it is desirable to measure the quantity of energy used, both per unit of time (e.g. power), and over a specific amount of time (for example, energy used per month). In a residential environment, an electric power meter is a familiar fixture; it allows the power-providing companies to charge their customers for the energy used.
In the most basic terms, the instantaneous power is a product of the voltage applied to, and the current flowing through, the load. An integration of this product (power) over a specific time interval yields the total consumption of the electrical energy within that time interval.
Accurate measurement of the load current is thus an important part of the apparatus that measure the consumption of electrical energy.
It will be helpful to review the present state of the art in current measurements, considering numerous examples of the devices capable of current measurement. In broad terms, they all fall into two categories, namely:                direct measurement of the current as manifested by a voltage drop across a sense resistor (oftentimes called a resistive shunt), and        indirect measurement of the flowing current by assessment of the magnetic field that is created around the conducting wire that is carrying the current.        
Current-measuring apparatus further differ in their ability to measure DC (direct current) or AC (alternating current), and also differ in the total range of frequencies that can be processed with a high degree of accuracy. The apparatus that can accommodate large currents also differ in their ability to measure small currents accurately.
Typical current sensors in industrial, commercial, and household environments furnish the AC measurements; the parameter of interest is usually the peak-to-peak or Root-Mean-Square (RMS) value of the current, averaged across at least a single cycle of the AC frequency; in such measurements the DC component of the current is assumed to be exactly zero. The ultimate use of these measurements is for an overall consumption of energy, and the sensor is made to be quite accurate when a large level of current is flowing. Errors resulting from inadequate precision of measurements for small levels of current are simply ignored, as they contribute only minimally to the total energy assessment. Stated differently, when one is measuring large currents to assess overall energy consumptions, one does not mind that small currents are not measured accurately.
A moment's reflection, however, will reveal that for operations with DC power, and specifically for DC-power systems that use a battery, the current measurement needs are quite different. It is not uncommon for a DC-supplied system to spend most of its time operating at a very low power level, and to consume the full-rated energy only for relatively brief intervals. In such a system, the current-sensing apparatus must be able to faithfully measure the current across widely different levels, while at the same time maintaining at most a very small DC offset error. In many DC power systems that use a battery, it is desired to monitor the SOC (state of charge) of the battery which requires continuous and uninterrupted current measurements so that integrations may be carried out, making coulometry possible.
Several contradictory constraints apply to such a current sensor, for example:                The output signal should accurately represent the measured current. In technical terms, the sensitivity or conversion gain between the measured current and output signal should be accurate and stable.        The readings for times when no current is flowing should be as close as possible to zero. In technical terms, the systematic offset of the measurement should be zero.        The output signal should be available continuously, with quick response time to the changes in the measured current.        The current sensor should be insensitive to interfering RF (radio frequency) and EM (electromagnetic) fields, because such fields are naturally present in any environment where electrical energy is used, and are typically generated by the very same apparatus that use the electric energy. This requirement usually dictates the use of so-called RFI (RF interference) filtering, in order to prevent the interfering signals from affecting the measurements.        The power dissipation in the sensing element should be minimal.        The energy consumed by the current measurement circuit should be small; this has an increased importance for a system that a operates from a battery, and the actual goal of making the current measurements is for reduction of the energy consumption and/or estimation of the remaining energy in the battery. Having said this, in many applications it is acceptable to permit the energy used in the current measurement circuit to increase somewhat when large currents are flowing, that is, when large amounts of energy are being consumed. In other words, it is desirable to have low energy consumption in relative terms, where the power consumption of the measurement circuit will not appreciably affect the total energy value.        
The most common method of current sensing is to pass the current through a resistor (a current shunt) and to measure the resulting voltage drop, which develops according to Ohm's law. A current sensor circuit based on this principle is illustrated in FIG. 1. input terminals 1 allow connection of the current shunt 2 into the circuit where current is to be measured. Instrumentation amplifier (IA) 6 senses voltage across the current shunt 2, and provides (voltage) output on terminals 10. Pick-up points 3 on the current shunt 2 are located following the principle of “Kelvin sensing” that reduces errors associated with resistance of the sense connections and wires. With such an arrangement, the conductors that do the sensing have almost no current, and so there is very little IR (current times resistance) drop in those conductors. RFI filter 4 is utilized for reduction of effects due to interfering RF signals from surrounding machinery and environment. This part of the circuit may have various configurations, and may consist of resistive, capacitive, and inductive components, as well as semiconductor transient protectors. The particular configuration 4 shown in FIG. 1 is exemplary but there are many possible arrangements which might be employed.
FIG. 1 shows an instrumentation amplifier 6 modeled as containing an ideal amplifier 7. The ideal amplifier 7 is imagined to have infinite input impedance, zero output impedance, linear gain, and zero offsets or drift, all across some defined dynamic range of inputs and from DC to at least some frequency defining some frequency response. Also shown in FIG. 1 are modeled sources of error 5, 8, and 9 that make the performance less than ideal when compared to the requirements above. The modeled voltage sensing error sources 5 appear in series with the proper signal, and introduce unpredictable offset errors at the inputs of the IA 6. These errors can be further modeled as illustrated in FIG. 2.
Turning to FIG. 2, items comprising a string of dissimilar materials 13, 14, 15, 17, 18, and 20 are joined by solder 16. Exemplary items may be a Kelvin connection lead 13 from the current shunt 2, a surface-mount component 15 such as a resistor, an IC (integrated circuit) lead 17, a lead-to-die bonding wire 18, and IC die metallizations 20 at the IC die 19. A printed circuit (PC) board 12 is composed in part of a substrate material such as FR-4. A modeled temperature gradient 21 is shown, with a higher temperature at one end of this string and a lower temperature at the other end of this string. As mentioned below, anisotropies and inhomogeneities of temperature may easily develop to be more complicated than a mere gradient as shown in FIG. 2. As will be discussed below in connection with the invention, approaches discussed below have some prospect of measuring and correcting errors due to temperature variations that are more complicated than mere gradients.
The materials depicted in FIG. 2 make up the connections between the current shunt 2 and the actual input terminals of the IA 6 (which are part of the die metallization 20). Redrawn in a more recognizable form at the lower part of FIG. 2, it will be appreciated that these connections are in fact thermocouples, each capable of producing a voltage when a temperature difference (temperature gradient 21) exists between the extremes of the circuit at points 13 and 20, or inside of the string of the serially connected elements. This error voltage could potentially be on the order of several millivolts (mV). The most common temperature-induced errors may be because of gradients between 13 and 20, but as will be appreciated, any anisotropies or inhomogeneities in temperature can likewise give rise to error voltages.
Returning to FIG. 1, the error source 8 is a modeled offset error in the IA 6, and the error source 9 is a modeled noise error in the IA 6.
As will be appreciated, the circuit is designed utilizing differential signals, and the designer will hope that this will compensate most of the thermocouple-induced errors, because such errors are to some extent common-mode in nature (affecting each line of the two differential inputs in somewhat the same way). But any non-identical temperature distributions as between the two paths can give rise to errors which are not automatically compensated by the use of differential sensing.
Because there are error sources 5 in the differential sensing lines, the designer of a system such as is depicted in FIG. 1 will often not go to the trouble to select a module or integrated circuit for IA 6 that has a particularly low offset voltage 8.
The circuits similar to FIG. 1, or derivatives thereof, are generally used for measuring currents up to perhaps 10 A (Amperes). The designer of the current sensing system will typically select a shunt value such that the voltage drop across the sensing resistor 2 is 100 mV or more, so that this voltage will swamp or overwhelm the error sources just described. For any given maximum rated current (that is, for some particular real-life application) the resistance of the shunt 2 will be chosen so that the voltage developed across it is within some range of voltages. This leads to a situation where the heat dissipated in the sense resistor 2 grows at least linearly with the rated maximum current for which the particular circuit is being designed.
The physical size of the sense resistor 2 can thus get to be a problem, as can be the need to providing adequate cooling of the sense resistor. For this reason, for higher currents a different class of current sensing devices is used, typically utilizing an indirect method of current sensing by assessment of the magnetic field that is created around the conducting wire which is carrying the current.
The so-called Hall-effect apparatus is common for current measurements in excess of several Amperes. A Hall-effect device is able to produce a (differential) voltage signal when a continuous (supply) current is sent through the device, and a magnetic field is present that is perpendicular to the flow of that supply current. The voltage signal in the Hall-effect device is linearly proportional to both the supply current and the magnetic field, within limitations of power dissipation resulting from the supply current, and some additional anomalous effects. The current that needs to be measured generates the magnetic field acting on the device.
The sensitivity of the Hall-effect device to the magnetic field depends, among many things, for example upon mechanical dimensions of the sensing element, material composition and uniformity, attachment of the electrodes, stability and accuracy of the supply current, and construction of the magnetic core that concentrates the desired magnetic field and rejects interfering magnetic fields (of which the Earth's magnetic field is just a single example).
Typically every single manufactured Hall-effect device would need an individual calibration in order to ascertain the actual device sensitivity (which further depends on the particular circuit arrangement and on the design of the magnetic path elements).
The native offset of the unaided Hall-effect device is nonzero. For this reason various auto-zeroing schemes have been utilized in the prior art. One approach is to arrange for the supply current to the sensing element to be AC rather than DC. Within such an approach, differing schemes make use of various shapes or waveforms of the AC current, for example sine waves or square waves or even more complex shapes. The AC signal that is the output of the Hall-effect sensing element is further processed by a synchronous detector.
In a system where the Hall-effect sensor is employed and where the current sensed is AC, it usually turns out to be necessary to average or filter the sensed signal over several cycles of the AC excitation. This means that there is always some latency between a current event of interest and the detection of such an event after the averaging or filtering has taken place. The latency cannot be reduced to zero.
A further potential difficulty with such magnetically coupled measurements (particularly where a DC current is converted to AC for purposes of Hall-effect sensing) is that during the zero crossings of the excitation voltage (that is, near the zero values for the sine-wave excitation), the Hall-effect device is altogether insensitive to the magnetic field, and simply discards any information for the duration of the zero-crossing transitions.
Yet another variation of the Hall-effect devices makes use of an active feedback loop that tries to zero-out the total magnetic field acting on the sensing element, thus reducing possible non-linearities in the sensing element (since in this case, the Hall-effect sensing element only needs to indicate if the magnetic field is smaller or higher than zero, and does not need to supply the actual value). A winding on the magnetic path elements creates a magnetic field that is opposite to the field from the measured current. The opposing winding is normally constructed with many turns (typically, with several thousands of turns), so as to minimize the current that needs to be injected into the winding in order to zero out the total magnetic field. The servo loop that drives the zeroing-out winding will of course take some nonzero time to settle and to respond to perturbations.
It will be appreciated that in addition to slow time response, the Hall-effect devices utilizing the active feedback will require additional operating energy in order to supply the feedback circuitry and winding. These factors make the Hall effect sensing less than ideal, particularly for a battery-powered system.
Other current measurement approaches have been devised that indirectly sense current by assessment of the magnetic field that is created around the conducting wire that is carrying the current. These are based on a so-called magnetoresistance (MR) effect, including variations called Giant Magnetoresistance (GMR) and Colossal Magnetoresistance (CMR). These devices rely on resistance changes within the sensing material due to the magnetic field to which the sensing material is subjected.
The error sources mentioned above with respect to shunt-type current measurements present themselves, mutatis mutandis, with the Hall-effect sensing approaches just discussed, and also present themselves with the apparatus based on the MR-effect. For the MR-effect approaches, the notably worst performance is in respect to the zero-current offset for the MR-effect based measurements.
It would be very desirable if approaches could be found for current measurement, particularly DC measurement, which would be accurate at low currents as well as high currents, and which would be continuously available. It would be desirable if most offset error sources could be zeroed out, leading to measurements accurate enough for coulometry. It would be desirable if the approaches could dissipate very little waste heat in the sensing apparatus, and if the measurements could simultaneously:                be low enough in latency to permit a quick-response electronic “fuse” capability, and yet        be sufficiently filtered against RFI so as to permit filtering out such interference.        
It would further be desirable if some or all of these results could be achieved economically, without the need for unnecessarily expensive parts such as unnecessarily expensive semiconductor switches and the like.